1. Field of the Invention
The present application relates to a new process for the preparation of multilayered, composite oxygen enrichment membranes for separating oxygen from air and gas separation membranes obtainable from the same process.
2. Background of the Prior Art
In non-porous polymer membranes, gases are dissolved at one surface of the membranes and, then, the gases so dissolved are diffused, depending on the concentration gradients thereof, to the opposite surface, at which the gases are permeated out. This phenomenon, known as the solution-diffusion mechanism, may be explained by Fick's first law as set forth below: ##EQU1## where J is the flux of the permeating gas per unit area; D is the diffusion coefficient; C is the concentration of the permeating gas inside the membrane; and X is the space coordinate indicating optional position inside the membrane.
The concentration, C, of permeating gas inside the membrance is proportional to the gas pressure, p. This phenomenon to which Henry's law is applied is expressed by the following equation: EQU C=Sp (2)
where S is the solubility coefficient or the Henry's constant.
In a steady state, since the concentrations at both ends of the membranes becomes constant, the flux also become constant accordingly, and Equation (1) may be rewritten to give the following equation: ##EQU2## where C.sub.1 and C.sub.2 are the concentrations of a permeating gas at high and low pressure sides of a given membrane, respectively, and L is the thickness of the membrane.
Equations (2) and (3) may be combined to give the following equation: ##EQU3## where P is defined as DS, which represents the permeability coefficient.
Another measure for the indication of gas permeation properties of membranes, in addition to the permeability coefficients, is the selectivity of membranes. The selectivity of membranes to component a from component b among a gas mixture consisting of components a and b is defined by the ratio of the permeability coefficient of component a to that of component b. This ratio is referred to as the ideal separation factor, Aab, and is defined by the following equation: ##EQU4##
Pa and A as are the constant values indicating intrinsic attributes of the materials from which the membranes are made, and are not influenced by methods for the production thereof. Given the materials for separation membranes, the selectivity becomes constant. Since the flux is inversely proportional to the thickness of membranes, it may be increased by reducing the thickness of the active layer of membranes. The thin membranes lack mechanical strength and, thus, it is necessary to use a supporting layer in order to compensate for the lack of the mechanical strength. In this purport, the composite membranes have been suggested which are produced by coating a material having good permeation properties on a porous supporting layer having good mechanical strength. Since the composite membranes are excellent in both the permeation properties and the mechanical strength, almost all of the separation membranes currently used adopt such composite membranes.
In many cases, the porous supporting layer of the composite membranes acts as a mechanical supporter, but occasionally it constitutes an active layer at which separation can occur. In the case of composite membranes, because it is difficult to measure the thickness of active layer, a new permeability is established and commonly used. The permeability, P, is defined by the following equation: EQU P=P/L (6)
Equations (4) and (6) are combined to give the following equation: EQU J=P(p.sub.1 -p.sub.2) (7)
As explained hereinbefore, the permeation property of composite membranes is usually represented by the permeability, P, and the ideal separation factor, Aab.
Heretofore, a number of patent applications relating to the production of composite membranes have been filed since such membranes are excellent in terms of both the permeation property and the mechanical strength.
For example, Henis et. al. U.S. Pat. No. 4,230,463 and Korean Patent Application No. 0171/1982 disclose a method for the preparation of multicomponent, composite membranes which are superior in both the permeability and the selectivity. According to this method, the membranes are produced by plugging pores on the surface of hollow fiber membranes made of polysulfones with silicon rubbers having good gas permeability. To yield composite membranes, a method is featured which applies a vacuum onto the inner surface of porous, hollow fiber polysulfone membranes so as to cause an occluding contact of silicon rubber with the polysulfone membranes. In this method, the size and the number of skin pores of the porous, hollow fiber polysulfone membranes should be exceedingly small in order to give composite membranes having excellent permeability and selectivity.
Riley et al. U.S. Pat. No. 4,243,701 teaches a process for the preparation of gas separation membranes, which comprises coating a silicon rubber solution having good permeability onto the surface of cellulose acetate or polysulfone membranes. The produced membranes have good permeability, but its selectivity is not high.
Cabasso et al. U.S. Pat. No. 4,602,922 teaches a process for the preparation of composite membranes, which comprises coating a silicon rubber solution having good permeability onto the surface of porous polysulfone membranes in the same manner as in U.S. Patent No. 4,243,701 to Riley, and further coating a modified polyphenylene oxide having good selectivity onto the above silicon rubber coating to give a composite thin membrane. However, this method comprising two steps of coating is not only inconvenient but also lowers the permeability.